1. Field of the Invention
The present invention relates to the measurement of S-parameters of a device under test (DUT). More particularly, the present invention relates to a measurement device such as a vector network analyzer (VNA) configured to measure the transmission coefficient and group delay of a bi-directional two port DUT.
2. Description of the Related Art
Traditionally, a VNA having two measurement ports has been used to determine the S-parameters and group delay of a two port DUT. The S-parameters consist of S.sub.11, S.sub.12, S.sub.21, and S.sub.22. S.sub.12 is the transmission coefficient for the first DUT port, and S.sub.21 is the transmission coefficient for the second DUT port. Each transmission coefficient is comprised of a magnitude and a phase angle. Group delay is the change in the transmission coefficient phase angle with respect to the change in the frequency of a signal traveling through the DUT.
When using two VNA measurement ports, the transmission coefficients and group delay are measured as follows. The first DUT port is excited by a first input signal from a first VNA measurement port, and a second VNA measurement port measures an output signal at the second DUT port. The second DUT port is excited by a second input signal from the second VNA measurement port, and the first VNA measurement port measures an output signal at the first DUT port.
The S.sub.12 transmission coefficient is determined by measuring the return loss and phase change in a signal at the second measurement port that was received through the DUT from the first measurement port. Similarly, the S.sub.21 transmission coefficient is determined by measuring the return loss and phase change in a signal at the first measurement port that was received through the DUT from the second measurement port.
Group delay for the DUT is calculated by measuring change in the phase angle for the transmission coefficients with respect to change in the frequency. Group delay is calculated by the following equation: EQU GD=-d.crclbar./d.omega.; (1)
wherein:
GD is the group delay PA2 .crclbar. is a transmission coefficient phase angle; and PA2 .omega. is a frequency of the input signal.
However, in many cases it is not feasible to connect both ports of a two port DUT to a single VNA having two measurement ports. One example of such a two port DUT is a coaxial cable being used in a local area network that is already installed in a building. Another example is a coaxial cable connecting a transmitter/receiver to an antenna. It may be desirable to test each cable in these examples, but the two ends of the cable may physically reside at locations a significant distance from each other. Consequently, it may not be possible to reliably connect both ends of the cable to a traditional two measurement port VNA for measuring the transmission coefficients and group delay of the cable.
Further, some measurement devices only include a single measurement port. Conventional techniques of measuring transmission coefficients and group delay are not available for such a device. Such a single port measurement device is disclosed in U.S. patent application Ser. No. 08/362,179, entitled "Handheld Vector Network Analyzer", now U.S. Pat. No. 5,642,039.
Accordingly, a measurement device that can measure the transmission coefficient and group delay of a two port DUT by only using a single measurement port would be desirable.